The present application relates generally to Voltage/VAR control optimization for power distribution systems. Power distribution systems are increasingly encountering the presence of distributed energy resources (DER) such as residential photovoltaic (PV) systems. High levels of DER penetration in power distribution systems present new planning and operation challenges to the system operator. DER systems can cause appreciable net power injections on distribution system as well as transmission systems. The injection of power on distribution circuits cause voltage rises on the distribution circuits which can become very severe for high levels of net power injections and may exceed prescribed standards or operating limits. Additionally, power from DER systems is frequently intermittent, uncertain and can change rapidly. The level of power from DER systems can vary significantly in a short period of time, causing rapid voltage variations in the distribution system. Traditional voltage regulation techniques are inadequate to handle the rapid voltage variations due to DER system power output changes. Furthermore, in many instances significant information about the power distribution system is unknown to the control center that manages the power distribution system. A number of aspects of the distribution system circuit topology may not be fully characterized or known. This obstructs the use of techniques requiring detailed a power flow model and/or a power flow solutions of the power distribution system.